1. Technical Field
The present invention relates to one-phase microemulsion compositions, O/W ultrafine emulsion external formulations, and the methods for producing the same, and in particular, relates to the simplification of the production method of the one-phase microemulsion compositions, the improvement of the safety of the compositions, and the improvement of the safety and stability of the O/W ultrafine emulsion external formulations that are obtained by adding the one-phase microemulsion composition to an aqueous formulation.
2. Background Art
In the past, ultrafine emulsion formulations have been used in various fields such as cosmetics, pharmaceuticals, agrochemicals, water-based paints, waxes, and food. It is known, in particular, that O/W ultrafine emulsion formulations are used in external formulations. The known preparation method of ultrafine emulsion formulations is a blending method of a one-phase microemulsion, which can be obtained according to the methods described below, into an aqueous formulation.
The first method is based on the phenomenon that when a hydrocarbon oil such as cyclohexane or cycloheptane is added to an aqueous solution of a high HLB nonionic surfactant and the temperature is increased, a region of drastic increase in the solubility of hydrocarbon oil appears just below the cloud point of the nonionic surfactant (refer to “Solution and Solubility” written by Kozo Shinoda, Maruzen Co., 1991, pp 209-225). In the one-liquid-phase region (Iw) between the solubilization limit temperature and the cloud point that are shown in the phase diagram, the solubility of oil in the water phase drastically increases, thus, a so-called “one-phase microemulsion” is formed. The one-phase microemulsion of a nonionic surfactant-hydrocarbon system, which has been studied in the past, is thermodynamically stable in a very narrow temperature range (ca. several ° C.-10° C.), where hydrophilicity and lipophilicity are balanced. However, the system will become cloudy and soon separate into two phases if the temperature deviates even slightly from this range. Thus, it has been very difficult to apply such one-phase microemulsions to cosmetics and pharmaceuticals.
In the second method, the hydrophilicity and lipophilicity are balanced by the combination of an anionic surfactant and a cosurfactant such as pentanol, hexanol, or octanol. The thus generated very narrow range where the solubility of hydrocarbon oil drastically increases is utilized. In the third method, a lipophilic nonionic surfactant and a specific ionic surfactant are combined, or a lipophilic nonionic surfactant and an ionic surfactant are combined and an electrolyte is added to this combination. In these compositions, there is a very narrow ratio range, where the hydrophilicity and the lipophilicity are balanced. This region where the solubility of hydrocarbon oil drastically increases is utilized (refer to: Kozo Shinoda and Hiroyuki Saijo, Yukagaku (Oil Chemistry), 35, 308-314 (1986); Japanese Unexamined Patent Publication S58-128311; and Japanese Unexamined Patent Publication S58-131127). The fourth method, in which a silicone oil, a silicone surfactant, and a hydrophilic surfactant are combined to balance the hydrophilicity and lipophilicity, is also known (refer to Japanese Unexamined Patent Publication H10-120524).
Although the temperature stability of the one-phase microemulsions obtained by these methods is high, the thermodynamically stable composition range of the obtained one-phase microemulsion is extremely limited. If the composition deviates from this range, white turbidity is caused and a phase separation takes place. Thus, the practical product formulation has been very limited or complicated. In addition, the blending of the ionic surfactant or cosurfactant has a problem as safety, irritation to human body.
A preparation method of an ultrafine emulsion by micronizing with a strong shearing force, emulsion particles in a fluid mixture that contains water and oil is commonly known (refer to Japanese Unexamined Patent Publication S63-12654 and Japanese Unexamined Patent Publication H01-293131). For example, dispersed particles are micronized with a Gaulin-type high-pressure homogenizer. In this method, a sample is pushed out of a narrow opening with a high pressure, and the cavitation and turbulent flow, which are generated upon transition to a normal pressure, is utilized to micronize dispersed particles. A new type of high-pressure emulsification apparatus was also proposed. However, if the emulsification pressure is set high for the emulsification apparatus, the base temperature increases during the treatment, thus often affecting the stability of the emulsion (refer to Japanese Examined Patent Publication H02-976526 and Japanese Unexamined Patent Publication H11-47580).
Although a treatment with an ultrasonic wave has also been proposed, large scale production with an ultrasonic wave has been difficult. Because the preparation of an ultrafine emulsion by a physical method necessitates a high energy, special equipment such as a high-pressure emulsification apparatus was necessary.